1. Field of the Invention
This invention provides a method and apparatus for producing a falling curtain of liquid. This particular method and apparatus has been developed to provide a technique for the application of starch solution to the plies of multi-ply paperboard and the invention also extends to methods and apparatus for the manufacture of paperboard using this technique.
2. Description of Prior Art
The paperboard industry is heavily committed to the use of wastepaper. More than half the total furnish of boxboards and container liner grades is reclaimed fibre. Included in any bulk collections of wastepaper there is a proportion of high yield pulps which cannot be separated economically and which adversely affect certain stock properties and starches are frequently employed to improve the performance of grades such as a container liner manufactured from such stocks.
The usual manner of applying starch to strengthen multi-ply paperboard is with the aid of a size press in the dryer section of the board forming machine. The partially dried multi-ply board is passed through the size press where it absorbs the starch solution. This operation also increases the moisture content of the board and adds to the drying load imposed on the remainder of the drying section of the paper board machine. In view of the marked increase in bursting strength achieved by the starch addition, however, paper mills are prepared to accept this additional drying load which reduces output.
The addition of 5% starch directly to the furnish would produce possibly 5 to 10% increase in bursting strength of the final product. The addition of further starch has little effect on strength. By comparison the application of 2 to 3% starch at a size press located in the drying section of a paper board machine can yield a 15 to 20% increase in burst strength. The starch additions in each case are calculated as a percentage of total fibre content. It has been observed however that the starch at the size press penetrates only the outer plies of the board and the gains in strength per ply actually penetrated may be more than twice that achieved with addition to the furnish. It follows that if the plies could be effectively treated with starch individually the total burst increment could be increased very significantly or alternatively advantage could be taken of the enhanced effectiveness of the starch to reduce the amount of raw material used, to increase productivity through higher machine speeds, or to make use of lower strength wastepaper.
During the formation of multi-ply paperboard on high speed formers such as the Inverform type, successive plies are exposed for a brief period as the board is built up which provides the opportunity to apply starch successively to the individual plies and the curtain coater of the present invention has been developed for this purpose. Its use is not limited to this particular purpose, however, and it may find application in other fields.